Light Reactions

Bozmeman Science “Photosynthesis & Respiration”

Light Reactions Summary: Light energy splits H2O to O2 releasing high energy electrons (e-) Movement of e- used to generate ATP Electrons end up on NADP+, reducing it to NADPH

Photosystem: reaction center & light-harvesting complexes (pigment + protein)

Photosystems of photosynthesis 2 photosystems in thylakoid membrane collections of chlorophyll molecules act as light-gathering molecules Photosystem II chlorophyll a P680 = absorbs 680nm wavelength red light Photosystem I chlorophyll b P700 = absorbs 700nm wavelength red light reaction center Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane. When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center. At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a. This starts the light reactions. Don’t compete with each other, work synergistically using different wavelengths. antenna pigments

What’s Happening? Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane. When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center. At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a. This starts the light reactions. Don’t compete with each other, work synergistically using different wavelengths

Electrons in chlorophyll molecules are excited by absorption of light

Electron Flow Two routes for electron flow: Linear (noncyclic) electron flow Cyclic electron flow

Light Reaction (Linear electron flow) Chlorophyll excited by light absorption E passed to reaction center of Photosystem II (protein + chlorophyll a) e- captured by primary electron acceptor Redox reaction  e- transfer e- prevented from losing energy (drop to ground state) H2O is split to replace e-  O2 formed

Light Reactions Cont’d. e- passed to Photosystem I via ETC energy transfer pumps H+ to thylakoid space ATP produced by photophosphorylation e- moves from PS 1’s primary electron acceptor to 2nd ETC NADP+ reduced to NADPH MAIN IDEA: Use solar E to generate ATP & NADPH to provide E for Calvin cycle

Turn & Talk: EXPLAIN what is happening Reaction Center = Chlorophyll a

Turn & Talk: EXPLAIN what is happening

Turn & Talk: EXPLAIN what is happening

Reaction Center = Chlorophyll b

Mechanical analogy for the light reactions

Cyclic Electron Flow: uses PS I only; produces ATP for Calvin Cycle (no O2 or NADPH produced)

Both respiration and photosynthesis use chemiosmosis to generate ATP

Proton motive force generated by: H+ from water H+ pumped across by cytochrome Removal of H+ from stroma when NADP+ is reduced